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Can you help with CYA accumulation calculation please

Dear Seniors,

The more I read, the more I realized I don't know many things on pool chemisty.
I am still waiting for my TF100 test kit and it will not come until sometime, because it will join my other shipment within USA into Indonesia.

Anyway, I am curious about having too much CYA in my pool or better I would say that I would like to be able to predict CYA accumulation in my pool.

I am now using 90% Tri-Chlor tablet at 3" size and 250 grams per tablet ( approx 8oz )

TPF sticky mentioned that for Tri-Chlor, it will add 0.6 part CYA for every 1.0 part of chlorine it adds.
Pool calculator for Tri-Chlor stated that 1PPM of FC in my 35K pool will need 5.1 oz ( 150 grams ). I am assuming that my Tri-Chlor being a 90% strength is the same as what the Pool Calculator is using.

I don't have my pool log book with me, but I am averaging off my head at 3kg ( 100 oz ) Tri-Chlor use a month. Sometime I add Cal Hypo for shock and that does not contain CYA.

To today from March 2007 is 14 months or 42kg ( 92.4lbs ) of Tri-Chlor used. I add water at about 400 liters per day or 105 gallons. So in total my pool water including losses is 14 months x 30 days x 400 liters = 168,000 liters. Pool size is 135,000 liters, grand total is = 303,000 liters ( 80,158 gallons ).

116 PPM of chlorine equivalent been consumed in my pool that has total water contained and consumed so far at 303,000 liter ( 80,158 GL ). If 0.6 of it is CYA and that will stay in my pool, I am assuming I am now having 116PPM x 60% which is 70 PPM of CYA.

These calculations can get confusing because the way things are measured in "ppm" isn't as one would expect. The rule that for every 1 ppm FC you get 0.6 ppm CYA when using Trichlor is independent of the concentration of Trichlor itself. It's just a chemical fact based on how much chlorine is attached to the CYA that makes up Trichlor and what happens when this dissolves in water.

Your 92.4 pounds of Trichlor in 35,000 gallons would result in cumulatively adding 289.6 ppm FC and 175.7 ppm CYA. So over 14 months that's around 0.7 ppm FC per day which is actually on the low side, but possible depending on sunlight exposure and maintained FC level (the usage in winter is probably very low and in summer is higher).

WARNING: This gets more technical below. Read on only if interested.

One 3" 8-ounce tablet of Trichlor will increase the FC by 1.57 ppm and the CYA by 0.95 ppm in your 35,000 gallon pool. The 90% number isn't the purity of the Trichlor (which is nearly 100%) but is the "Available Chlorine". This basically says that Trichlor produces 90% of the amount of chlorine as the same weight quantity of chlorine gas. The actual available chlorine is closer to 91.5% for pure Trichlor. The "ppm FC" and indeed all ppm chlorine measurements are relative to chlorine gas. 35,000 gallons of water weighs about 35,000x8.33 = 291,500 pounds so 0.5 pound of Trichlor would give 1,000,000*0.5/291,500 = 1.71 ppm Trichlor which gives 1.7*91.5% = 1.57 ppm FC. This is roughly what The Pool Calculator uses in its calculations. The CYA is just 0.607 times that amount so 1.57*0.607 = 0.95 ppm CYA.

Essentially, your "Version 1" is incorrect because you can't look at Trichlor as being 90% chlorine and 10% CYA. It is true that the Available Chlorine can be used to determine the chlorine amount, as ppm FC, as shown above, but it is incorrect to use the "balance" for determining the CYA amount. Trichlor is essentially 55.5% CYA equivalent by weight. Note that 55.5%/91.5% = 0.607 which is that ratio of CYA per FC. The reason these numbers don't add up to 100% is that there are THREE chlorine in Trichlor and ALL of these produce chlorine in water while there are TWO chlorine in chlorine gas while only ONE of these produces chlorine in water. One could theoretically have > 100% Available Chlorine in a product if it had mostly chlorine atoms that all produced chlorine in water (because the chlorine gas "standard" only has half of its chlorine atoms produce chlorine in water -- the other atom produces chloride ion). Trichlor has 45.76% chlorine atoms by weight and you'll notice that doubling this number results in 91.5% which is the Available Chlorine (that's a general rule: doubling the weight percentage of chlorine atoms that produce chlorine in the water gives the % Available Chlorine). The reason that the 55.5% plus the 45.76% don't add up to 100% is that the 55.5% is a CYA equivalent that includes hydrogen atoms replacing the chlorine and aren't actually in Trichlor. The CYA core that is actually in Trichlor without the hydrogen atoms is 54.25% of the weight of Trichlor and 54.25%+45.76%=100% ignoring rounding errors.

Your calculation regarding the amount of water you add is incorrect if such water is added to replace water lost by evaporation. Evaporation concentrates all chemicals in the water and adding fill water simply dilutes it back to where it was so there is no net change in concentration. Only actual water removal and replacement by splash-out, backwashing, or partial/full drain/refill will dilute the water.

This is the kind of answer I expect, thank you so much. I am blind on chemistry in school, never liked it but your explanation I can absorb.

My loss of 105 gallons per day is not mainly to evaporation, its the failure of the contractor to waterproof some part of the pool.
I don't know the percentage loss to usual evaporation though eventhough I know the rule of thumb, but I know its "leak" position. Its the area of the pool perimeter surface floor leading into the overflow gutter. Never mind.

OK, my pool is shaded, on left side it is 100% covered by 3 storey building, on rear end it is also 100% covered by a 2 storey building. On right side it is covered by a classroom, at least 60% of the pool length. The front of the pool has a wall of 6 feet at least. Direct sunlight is minimal, all I got is reflection. 1 of 3" Tri-Chlor can last 3 days in my pool with reading of 1.5PPM average.
That may explains my low CL consumption I guess and once a week or when bather load is high I dump that Cal-Hypo about 7 oz. Now I switch to 12% bleach for the last 20 days or so.

So interesting this pool thingy. I am glad I found TPF, people here are so helpful and many are very knowledegable people like yourself.